The 404 error messages will display when a page is not available for display.
If you believe the page should be available, please be sure you logged into Client Portal from the IBKR webpage as opposed to accessing it from TWS. If you still see the error in Client Portal, first clear your cache and cookies on your browser. If the issue persists, please try using a different web browser than the one you used initially to access the page.
If you continue to receive the 404 error message then please contact IBKR Client Services via ticket for additional troubleshooting. Please select Technical Assistance as the ticket category followed by Client Portal and type "404 Error" as the subject.